The FBI has joined the hunt for a suspected serial killer (above) who has murdered three Middle Eastern shopkeepers in Brooklyn, sources told The Post yesterday. (AP)

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The FBI has joined the hunt for a suspected serial killer who has murdered three Middle Eastern shopkeepers in Brooklyn, sources told The Post yesterday.

The development came as the NYPD released surveillance photos of four passers-by whom cops said they want to question in the most recent slaying, Friday. Authorities said the two men and two women are not considered suspects but were near the Flatbush store before owner Rahmatollah Vahidipour’s body was found inside at 7:11 p.m.

Federal agents in New York began assisting the NYPD after ballistics tests showed that the same .22-caliber handgun was used in all three slayings, one law-enforcement official said yesterday.

The agents have not called in serial-killer experts from the FBI’s famed lab in Quantico, Va., the source said, adding, “It’s still very early in the FBI’s involvement.’’

The people cops want to speak to were “identified’’ as:

* “John Doe Duffle Bag,” a balding man with a mustache who wore a long overcoat and carried a duffel bag. He was seen in the vicinity just after 6 p.m.

* “Jane Doe Long Coat,” a woman wearing glasses and a neck scarf who also was in the area at around the same time.

* “Jane Doe Green Jacket,” a younger woman with a white head scarf and green coat, who was seen running about a half-block from the scene while shedding her jacket at 6:52 p.m. The coat was found in a nearby garbage can.

* “John Doe Bubble Jacket,’’ a young man who was seen running in front of the woman with the green jacket.

The killer appears to do his homework. None of the businesses was heavily trafficked or had a video camera. Each murder was committed while the owner was closing by himself.

Cops were still investigating whether the killer might have stalked his prey at least partly based on the addresses of the stores. All had the numberal 8 in them.

Vahidipour, a 78-year-old Iranian Jew, was shot three times and his midsection covered with clothes.

But it did not appear the killer was trying to hide the body — it was “sources said.“It wasalmost like it was done in disgust” of the victim said one law-enforcement source.

Police revisited relatives of the second victim, Isaac Kadare, 59, an Egyptian Jew, who was shot in the head and stabbed in the neck in his Bensonhurst shop Aug. 2.

“They said they don’t have any leads, but hopefully they’ll catch him,” said Kadare’s widow, Nancy Kadare, 49.

The first victim, Mohammed Gebeli, 65, an Egyptian Muslim, was fatally shot in the neck in his Bay Ridge store July 6.

Cops dug into the backgrounds of the first two victims after linking their murders through ballistics. They even contacted Interpol to check if either had given any money to terrorists, law-enforcement sources said. They hadn’t.Additional reporting by Jamie Schram and Reuven Fenton